You are most likely using the old Crysis 3 MP Alpha client from the previous beta event. This client was connecting to different servers, so basically it's not working anymore.
Please make sure to download the new Crysis 3 MP Open Beta client. To do this, please switch to the Store in your Origin client and click on Demos at the top bar. You should see the Crysis 3 MP Open Beta and you can start the download and installation with a click on "Get it now".